As packaging materials, there are generally used biaxially-stretched films such as polypropylene film and polyethylene terephthalate film. These films, however, are poor in tear properties.
There is also known scored films to improve the tear properties of the films. Still, when a pouch is formed with a scored film, an article is affected adversely by the exterior atmosphere, or the exterior atmosphere is adversely affected by an article, in some cases, depending on the species of the content article.
On the other hand, in order to provide a readily openable packaging bag, there are also known films which are subjected to stretching in one direction to afford tear properties thereto. For example, Japanese Patent Application Laid-open Nos. 125844/1986 (61-125844), 208349/1987 (62-208349), 242746/1990 (2-242746) and 258342/1990 (2-258342) disclose a packaging material wherein a film extensively stretched in one direction is laminated on a paper or film as a base material.
Japanese Patent Application Laid-open No. 22537/1989 (64-22537) discloses a method of producing an easy tearable film which comprises mixing or blending olefinic polymers such as a low-density polyethylene or a high-density polyethylene in a predetermined proportion, extrusion-molding the mixture and stretching the resulting film in the radial direction to provide a flexible film having an islands-sea structure. This film, however, not only is poor in heat resistance and mechanical strength but also has a large tear resistance when torn by hand.
Japanese Patent Application Laid-open No. 180976/1990 (2-180976) also discloses an easy tearable film. However, a copolymer of ethylene component and a cyclic olefin of a specific structure is required to produce such a film and the film is poor in heat resistance.
Japanese Patent Publication Nos. 17632/1978 (53-17632) and 35832/1978 (53-35832) disclose a directly tearable film comprising polypropylene and an ethylene-.alpha.-olefin copolymer or others.
As thus described, when tear properties are imparted to a film by means of stretching treatment, a stretching step for stretching the film in one direction is required, thus the resultant film is high-costed and can be torn only in one direction. Further, even when the film is subjected to the stretching treatment, it is occasionally hard to tear without forming notches, and whiskers or palpi are branched from the film with tearing operation and thus the film cannot be torn sharply in one direction. Additionally, such stretching treatment may cause the formation of wrinkles or rumples by heat-sealing.
Japanese Patent Application Laid-open No. 93351/1989 (1-93351), Japanese Patent Application Laid-open No. 43047/1990 (2-43047) and Japanese Patent Application Laid-open No. 47038/1990 (2-47038) disclose a readily-cuttable polyester film having a tear propagation resistance of 300 g/mm or less in one direction. In this film, however, there is still required a copolyester having a specified structure.
Japanese Patent Application Laid-open No. 299831/1989 (1-299831) discloses a non-stretched or uniaxially-stretched film having oriented or directional tear properties which contains a polyolefin principally comprising an olefin having 4 carbon atoms or less, and polymethylpentene. Yet such an unstretched film is poor in tear properties, and stretching treatment is needed to improve the tear properties. Further, the specified polymer is still required to insure improved tear properties and, since all the above-mentioned polymers are olefinic polymers, the resulting film is still poor in, for example, heat resistance.
Japanese Patent Application Laid-open No. 19137/1992 (4-19137) discloses a method of producing an easy tearable multi-layer film which comprises extrusion-molding a resin compositions comprising two or more different and incompatible resins having largely different melting points each other to form numerous phases of the different resins in a certain direction. In this literature, there is described, as reasons of rendering the film to be multi-layer, that when a mono-layer film is composed of a resin having a higher melting point such as nylon and an olefin resin having a lower melting point in combination, the different resins are oriented in the drawing direction in a bundle form, thus the tear strength of the film is significantly sacrificed and the thus-obtained film is not practically usable. In such a film obtained by this technology, resins to be used are restricted to those having, for instance, largely different melting points from each other and a tearing direction is also limited to the predetermined direction with insufficient transparency.
Accordingly, it is an object of the present invention to provide an easy tearable film which can be torn easily at least in one direction without stretching treatment and a method of producing the same.
It is another object of the invention to provide an anisotropic easy tearable film which is tearable with an extreme ease in one direction and has a high tensile strength and tear strength in the orthogonal or transverse direction relative to the direction and a method of producing such a film.
A still another object of the present invention is to provide an easy tearable film having a practically usable strength and tearable in any direction and a method of producing the same.
A further object of the invention is to provide an easy tearable film which can be produced by use of conventional thermoplastic resins without a specialized polymer and a method of producing such a film.
It is still further object of the present invention to provide an easy tearable film being excellent not only in tear properties but also in transparency and a method of producing the same.